1. Field of the Invention
This invention relates to the treatment of fluoride-containing aqueous solutions to recover the fluoride values thereof in the form of aluminum fluoride.
2. Prior Art
Many industrial processes produce gaseous effluents containing hydrogen fluoride and other fluoride compounds. These gaseous effluents must be treated to remove the fluorides therefrom before the gas is discharged to the atmosphere and also to recover the fluoride values of the gas to avoid uneconomic waste of the fluorides. The most common procedure for recovering the fluoride content of effluent gases is to pass the gas through a scrubbing tower or other apparatus where the gas is scrubbed with water to remove fluorides therefrom and produce an aqueous solution containing these fluorides. The fluoride-containing solution is then, in most cases, treated to recover the fluoride values therefrom.
A number of processes have been proposed which follow this general procedure, the various processes using various scrubbing liquids or reagents to recover the fluoride values in forms suitable for various purposes. Most of these processes, however, require the concentration of fluorides in the scrubbing liquid to be relatively high (in the order of 10% or more HF or equivalent) in order for these processes to be effective and/or economical. Difficulty is encountered when the concentration of fluorides in the effluent gases and hence in the scrubbing liquid is relatively low, as is the case with the fluoride-containing gases evolved from the electrolytic cells in which alumina dissolved in molten cryolite is reduced to metallic aluminum.
My copending application Ser. No. 335,494 describes a process for the recovery of aluminum fluoride from dilute fluoride-containing aqueous solutions in which the fluoride content of the solution is reacted with alumina trihydrate (Al.sub.2 O.sub.3.3H.sub.2 O) at an elevated temperature to produce a complex aluminum hydroxide fluoride hydrate compound in the solution. Aluminum oxide (Al.sub.2 O.sub.3) is then introduced into the solution to crystallize aluminum hydroxide fluoride hydrate (16Al(OH,F).sub.3.6H.sub.2 O) from the solution, the precipitate being recovered and calcined to produce an aluminum fluoride product. The process results in the recovery of a significant portion, and in some cases up to 95%, of the fluoride content of the initial aqueous solution, and it may be used to recover a significant portion of the fluoride content of solutions containing as little as about 0.02% fluoride. In the practice of this process it has been found that equally satisfactory results are obtained when the aluminum oxide is added to the aqueous solution after substantial completion of the reaction between the fluoride content of the solution and the aluminum oxide trihydrate, or when the aluminum oxide is added to the aqueous solution before completion of the reaction between the fluoride content of the solution and the aluminum oxide trihydrate added thereto, or when both the aluminum oxide trihydrate and the aluminum oxide are added to the aqueous solution at the same time so that both the reaction and the precipitation of the reaction product proceed more or less simultaneously.
I have now made the surprising discovery that the fluoride content of dilute aqueous solutions of fluorides can be recovered in the form of the aforementioned aluminum hydroxide fluoride hydrate precipitate (16Al(OH,F).sub.3.6H.sub.2 O) when certain "commercial grade" aluminum oxides are employed as the reactant for the fluoride content of the solution, and that acceptable conversion and recovery of the fluoride content of the solution is obtained without prior reaction of the solution with alumina trihydrate Al.sub.2 O.sub.3.3H.sub.2 O) as required in the process of my copending application Ser. No. 335,494. Specifically, I have found that when commercial grade Bayer process aluminum oxide (comprising predominantly alpha alumina) having a loss of weight on ignition (as hereinafter described) of from about 0.5 to about 2.5% is reacted with the fluoride content of a dilute aqueous solution of fluorides under the conditions described herein, up to about 95% of the fluoride content of the solution may be recovered in the form of the aforesaid aluminum hydroxide fluoride hydrate precipitate.